Using the kinematic wave theory, a dimensionless expression is obtained, and solved numerically in order to estimate the lag time for diverging surfaces. The findings show that the dimensionless lag time, which is the ratio of lag time for diverging surface to that of plane surface, ranges from 1 to 0.5 depending on the degree of divergence and friction formula used. The results can be used easily and reliably for all ranges of the geometric parameter for urban and rural catchments.